EXAMINATION AND PRESERVATION MEDIA. 281 



refractive index surpassing that of any known resinous 

 medium. 



See further details in previous editions. 



436. Monobromide of Naphthalin. See Joum. Roy. Hie. Soc., 

 -1880, p. 1043 (ABBE and VAN HEURCK), and Zool Anz., 1882, p. 555 (MAX 

 FLESCH). 



437. THOMPSON'S High Refractive Medium. See Journ. Roy. Mic. 

 Soc,, 1892, p. 902. 



Resinous Media. 



438. Resins and Balsams. Resins and balsams consist of a 

 vitreous or amorphous substance held in solution by an essen- 

 tial oil. By distillation or drying in the air they lose the 

 essential oil and pass into the solid state. It is these solidi- 

 fied resins that should, in my opinion (and that, I believe, of 

 the best microscopists), be employed for microscopical pur- 

 poses ; for the raw resins always contain a certain proportion 

 of water, which makes it difficult to obtain a clear solution 

 with the usual menstrua, is injurious to the optical properties 

 of the medium and to its preservative qualities, and, further, 

 especially hurtful to the preservation of stains. I therefore 

 do not share the contrary opinion expressed by FOL (Lehrb., 

 pp. 138-9), but recommend that all solutions be made by 

 heating gently the balsam or resin in a stove until it becomes 

 brittle when cold, and then dissolving in an appropriate 

 menstruum. Solid resins are now easily found in com- 

 merce. 



Solutions made with volatile menstrua, such as xylol and 

 chloroform, set rapidly, but become rapidly brittle. Solutions 

 made with non-volatile media, such as turpentine, set much 

 less rapidly, and pass much less rapidly into the brittle state. 

 The former should, therefore, be employed whenever it is 

 desired to have a mount that sets hard rapidly ; but the 

 latter should be employed whenever it is above all desired to 

 have a mount that will prove as durable as possible. 



According to my experience, there is no auch thing as a 

 faultless resinous mounting medium for histoloyical purposes. 

 Solutions of gum damar in xylol are very beautiful from the 



